In recent years, increasing attention is attracted to the use of a water vapor permeable membrane for humidification or dehumidification. These humidifiers and dehumidifiers incorporating a water vapor permeable membrane have advantages such as being free from maintenance work and power sources for driving them.
Hollow fiber membranes, i.e. water vapor permeable membranes with a hollow structure, are used, for instance, to humidify the barrier membranes of fuel cell stacks. Fuel cells designed to be mounted on automobiles requires humidification for a very large flow volume of air of about 4,000 NL/min. Therefore, hollow fiber membranes for humidification are required to be high in water vapor permeability and hollow fiber membrane strength. Hollow fiber membranes for humidification also need to have gas barrier capability to prevent air leak from hollow fibers, as well as water vapor permeability. To this end, these hollow fiber membranes have a porous structure containing voids with very small diameters and a required water vapor permeability is achieved by applying a pressure. The required air flow volume largely varies depending on the road conditions and the way of driving. For instance, vehicles traveling on urban roads require only a small flow volume, while a large flow volume is required when they are running up a mountain road or accelerated suddenly.
There have been several proposals of hollow fiber membranes that cut off air while selectively allowing water vapor to permeate them.
A wide variety of polymers have been proposed as material for hollow fiber membranes for humidification. An example is a hollow fiber membrane for humidification comprising polyimide resin as membrane material. This membrane is characterized by high heat resistance, high durability and good gas barrier properties. However, it has the disadvantage of poor water vapor permeability.
Hollow fiber membranes for humidification comprising a fluorine-based ion exchange membrane are higher in water vapor permeability and gas barrier capability than hollow fiber membranes for humidification comprising polyimide resin. However, they are not high enough in water vapor permeability to serve practically as hollow fiber membrane for humidification, and they are also low in heat resistance. Furthermore, the hollow fiber membranes themselves will be very expensive.
Hollow fiber membranes for humidification comprising polyetherimide resin have recently been proposed. These are designed to be as high in water vapor permeability as fluorine-based ion exchange membranes, and also high in heat resistance.
In any case, the existing hollow fiber membranes for humidification are intended to have good gas barrier properties, but accordingly they are poor in water vapor permeability, failing to serve adequately as material for humidification.
In a recent proposal concerning membrane material, a spinning solution consisting of polyphenylsulfone resin and hydrophilic polyvinyl pyrrolidone dissolved in a water-soluble organic solvent is used with an aqueous N-methyl-2-pyrrolidone solution as core liquid, and subjected to dry-wet spinning to produce a hollow fiber membrane of porous polyphenylsulfone resin (Patent document 1). It is described, however, that the hollow fiber membrane thus produced is intended to serve as ultrafiltration membrane for oily water separation, and therefore, it is not designed for water vapor permeation.
There is another proposal for a polyphenylsulfone based hollow fiber membrane for humidification (Patent document 2). However, this hollow fiber membrane cannot achieve sufficient humidification performance.
There is another proposal for a polysulfone-based hollow fiber membrane for humidification in which the hollow fiber membrane has an asymmetric structure in which the voids located near one membrane surface have different diameters from those of the voids located near the other membrane, instead of having a uniform size (Patent document 3). However, such an asymmetric structure alone cannot act to develop a sufficient water vapor permeability and will fail to achieve a high humidification performance.
With respect to the relation between the number of voids in a hollow fiber membrane and its strength, on the other hand, the strength of the hollow fiber membrane decreases if the number of voids is increased to enhance the water vapor permeability, while the water vapor permeability decreases if the number of voids is decreased to enhance the strength of the hollow fiber membrane, thus resulting in a relation that requires a trade-off. To solve this problem, the use of a textured yarn commonly called covering yarn to cover a hollow fiber membrane (Patent document 4) and the use of high-strength rods to protect the external face of a hollow fiber module (Patent document 5) have been proposed. The method proposed in Patent document 4 actually can protect part of the hollow fibers by covering the hollow fibers with textured yarns, but it is difficult to increase the strength sufficiently to resist a high flow volume of 4,000 NL/min. In the method proposed in Patent document 5, high-strength rods act too strongly to restrain the gas flow into hollow fibers, leading to performance problems such as inefficient water vapor permeation.
In another proposal, covering yarns as described in Patent document 4 and a high-strength rod as described in Patent document 5 are used together by combining the high-strength rod and the hollow fiber membranes with the covering yarns (Patent document 6). This method is difficult to carry out in an in-line step in the hollow fiber membrane production process, making it necessary to wind the hollow fiber membranes around the high-strength rod in an off-line step, which requires very lengthy operations. Therefore, it is not suitable for a step that handles several thousands to several tens of thousands of hollow fiber membranes. Furthermore, a method in which hollow fiber membranes are arranged around a support yarn to achieve a required strength has been proposed (Patent document 7). This method, however, uses a low-stretchability support yarn passing through the center of a bundle of hollow fiber membranes, which fail to achieve a sufficient resistance to the high flow volume of gas in humidifiers.